The present invention relates generally to random access memory for data storage. More specifically, the present invention relates to a magnetic random access memory device that includes improved unidirectional elements to limit leakage current within the array.
Magnetic random access memory (MRAM) is a non-volatile memory that shows considerable promise for long-term data storage. Performing read and write operations on MRAM devices are much faster than performing read and write operations on conventional memory devices such as DRAM and flash and order of magnitude faster than long-term storage device such as hard drives. In addition, the MRAM devices are more compact and consume less power than other conventional storage devices.
A typical MRAM device includes an array of memory cells. Word lines extend across rows of the memory cells and bit lines extend along columns of the memory cells. Each memory cell is located at a cross point of a word line and a bit line.
A memory cell stores a bit of information as an orientation of magnetization. The magnetization of each memory cell assumes one of two stable orientations at any given time. These two stable orientations, parallel and anti-parallel, represent logic values of “0” and “1”.
The magnetization orientation effects the resistance of a memory cell such as a spin-tunnelling device. For instance, resistance of a memory cell is a first value R if the magnetization orientation is parallel and resistance of the memory cell is increased to a second value R+ΔR if the magnetization orientation is changed from parallel to anti-parallel. The magnetization orientation of a selected memory cell and, therefore, the logic state of the memory cell may be read by sensing the resistance state of the memory cell. The memory cells thus form a memory array of resistive cross points.
Applying a voltage to a selected memory cell and measuring a sense current that flows through the memory cell one may sense the resistance state. Ideally, the resistance would be proportional to the sense current.
Sensing the resistance state of a single memory cell in an array, however, can be unreliable. All memory cells in the array are coupled together through many parallel paths. The resistance seen at one cross points equals the resistance of the memory cell at that cross point in parallel with resistances of memory cells in the other rows and columns of the array.
Moreover, if the memory cell being sensed has a different resistance due to the stored magnetization, a small differential voltage may develop. This small differential voltage can give raise to a parasitic current, which is also known as leakage current. The parasitic or leakage current becomes large in a large array and, therefore, can obscure the sense current. Consequently, the parasitic current can prevent the resistance from being sensed.
Unreliability in sensing the resistance state is compounded by many factoring variations, variations in operating temperatures, and aging of the MRAM devices. These factors can cause the average value or resistance in the memory cell to vary.
The prior art has attempted to reduce leakage current through various designs. One approach involves adding a unidirectional element, such as a diode, to limit the current path in one direction. FIG. 1 illustrates such an embodiment. A MRAM device 1 comprises several rows 2 (bit lines) and columns 3 (word lines) which form an array having several cross points 4. At each cross point 4 a memory cell 5 is provided. Further, at each cross point 4, a diode 6 being connected to the memory cell 5 is provided. The memory cell 5, together with the diode 6, forms a conductive path between one row 2 and one column 3. The diode 6 limits current flow in one direction.
In order to achieve low leakage currents, the quality of the diodes 6 must be very high. However, high quality diodes are difficult to produce. In particular diodes being manufactured using polysilicon deposition processes are known as leaky diodes.
Accordingly, there is a need to provide a MRAM storage device having isolation diodes which show only a very small leakage current.